1. Field of the Invention
The invention relates to the art of crystal growing under controlled temperature conditions for the purpose of producing crystals with minimum number of defects.
2. Background Art
The discovery of high temperature superconductors above the liquid nitrogen temperature of 77.degree. K. paves the way for numerous practical applications. The Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 (123) and Bi.sub.2 Ca.sub.2 Sr.sub.2 Cu.sub.3 O.sub.10 (Bi2223) superconductors with critical temperatures of 90.degree. and 110.degree. K., respectively, are only two of an entire family of ceramics yielding a high temperature superconductivity. Since the discovery of of the first high temperature superconductor in 1987 above 77.degree. K., superconducting polycrystalline pellets, oriented films, and small size bulk crystals have been produced in laboratories. The films and bulk crystals have oriented structures allowing for greater critical currents and magnetic fields than the polycristalline superconducting materials.
The Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7 is typical of the superconducting perovskites and the inherent difficulties in manufacturing it are closely related to other members of the family. As such, the crystal manufacturing methods to be developed for this superconductor should be readily extendable to other high temperature superconductors. The bulk material processing problems of these superconductors are associated with incongruent melting, melt nonstoichiometry, crystal growth anisotropy, and the control of the processing environment such as oxygen pressure and crucible type. Because of incongruent melting and anisotropy of grown crystals, the most effective crystal processing methods appear to be growth from the melt and a solution. The crystal growth of 123 and Bi2223 compounds from the melt involve growth from a CuO rich flux in ZrO.sub.2 and ThO.sub.2 crucibles by a slow cooling process where the flux is used to reduce the melting point temperature. The critical crystal growth conditions consist of the melt composition, temperature distribution and cooling rate in the vicinity of the melt/crystal interface, temperature distribution in the crystal, and crystal annealing time and thermal cycling in an oxygen atmosphere. To achieve progressive crystallization of the melt in the vicinity of the crystal/melt interface it is necessary to remove the liberated latent heat and segregated solute from the interface. These processes occur by the heat diffusion in the crystal and by multicomponent mass diffusion and convection in the melt and solution, with additional complications produced by the crystal growing apparatus and radiation heat transport process. The growth of superconducting crystals requires, therefore, a cooling of the crystal to remove the latent heat and removal of the rejected solute from the interface region into the bulk of the melt or solution. The techniques of single crystal growth should, therefore, make provisions for removal of latent heat from the growing crystal and allow for some type of convective mixing in the melt or solution.
The production of high quality superconducting crystals thus requires the minimization of apparatus vibrations and thermal oscillations in the melt or solution and in the crystal. Thermal oscillations and constitutional supercooling in the melt can produce defects leading to low critical currents and magnetic field, or low quality crystals. Moreover, the induced stresses in the crystal can produce crack nucleation and deleterious effects on crystal properties. For these reasons, the superconducting crystal growing apparatus should be designed with the following characteristics: (1) minimization or absence of vibrations, (2) with a control of thermal fluctuations or temperature in the melt and crystal of better than 1.degree. C. from 600.degree.-1000.degree. C., and (3) minimization of induced thermal stresses. To minimize the crack nucleation in the neck of grown crystals and provide a constant diameter shape, the crystal can be grown by simply placing the material to be solidified in a cylindrical container and growing from a seed crystal at the bottom, with the bottom of the container or crucible maintained below the melting temperature. The elimination of vibrations can be achieved by a furnace design involving no moving parts, and the heat zone should be designed such as to impose on the solid and melt a thermal field whose isotherms are parallel with the bottom of the crucible and have an upward gradient.